This invention relates to a low actuating force switch, and more particularly to a low actuating force motor starting switch in which the deflection of one or more of the switch arms in the switch is limited so as to reduce the stresses applied to the switch arms.
Conventionally, an electric motor, such as a split phase or a capacitor start motor, includes an auxiliary starting winding in the stator assembly of the motor which is energized only during start up of the motor so as to provide sufficient start up torque for the motor. Once the motor has accelerated to a predetermined operating speed (e.g., 80 percent of its synchronous speed) the main windings of the stator assembly will produce sufficient torque to operate the motor. As is conventional, the motor includes a centrifugal actuator responsive to the motor speed and a switch, referred to as a motor starting switch, actuable by the centrifugal actuating member for deenergizing the starting winding once the motor has attained its predetermined operating speed. Upon slowing or stopping of the motor, the centrifugal actuator effects the closing of the motor starting switch so that upon reenergization of the motor, the starting winding will again be energized.
As is typical, a motor starting switch may have a fixed contact, a movable cantilevered switch arm, and a movable contact carried by the switch arm. The switch arm is cantilevered supported at one end by the switch housing and is flexibly, resiliently movable (i.e., bendable) by a switch operating member or plunger which is in turn moved along a path within the switch housing by the centrifugal actuator. The plunger is typically moved through a distance somewhat greater than is required to close the switch contacts upon the centrifugal actuator moving from its run to its off or stopped position. It is, in many cases, necessary that the actual stroke of the centrifugal actuator as it moves between its run and off position be greater than the minimum stroke required to close the contacts so that the relative position of the motor starting switch and the centrifugal actuator are not so critical. By having the stroke of the centrifugal actuator greater than is required, the operation of the motor starting switch is less sensitive to its position relative to the centrifugal actuator and thus the switch can accommodate a greater degree of end play of the rotor assembly in the bearings of the motor and of mispositioning of the switch relative to the centrifugal actuator.
Oftentimes the centrifugal actuator is capable of applying forces to the switch arm via the plunger far in excess of the force needed to close the contacts and to maintain good electrical contact therebetween. Because the plunger may be moved through a distance greater than is required to close the contacts and because it may exert relatively high force on the switch arm, the switch arm is in many cases (depending on the relative adjustment of the motor starting switch in the centrifugal actuator) subjected to high bending stresses when it is in its closed position upon each cycle of the motor starting switch as the centrifugal actuator moves from its run to its off position. Thus, the switch arms of certain prior art motor starting switches have, on occasion, failed due to fatigue. Of course, such a failure of the switch arm of a motor starting switch would prevent the motor from starting.
In certain instances, motor starting switches include other contacts operable by the centrifugal actuator for opening and closing other circuits upon start up and shut down of the motor. For example, the starting switch for the drive motor of an electric clothes dryer may include a set of contacts which open upon stopping of the motor so as to disconnect the dryer's heating elements from the power input lines to the dryer when the dryer is off. However, these additional sets of contacts require that the centrifugal actuator apply a considerable force above and beyond the force necessary to operate only the switch arm and contacts controlling the starting winding of the motor. If the force required to operate the various sets of contacts in the motor starting switch exceeds the force applied to the operating member by the centrifugal actuator, the contacts will not be properly opened and the motor and other elements of the appliance will fail to operate. This condition is sometimes referred to as a "stuck actuator" malfunction. Many prior art motor starting switches had relatively high actuation forces thus making operation of the centrifugal actuator even more critical.